Server 2025 High CPU

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mubbasher.hussain

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Jun 24, 2026
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Hi all,

I am currently testing Proxmox as a potential VMware replacement and have built a new Windows Server 2025 VM from ISO. I am experiencing unusually high CPU usage inside the VM.

This is not a migrated VM! it is a clean installation of Windows Server 2025.

The VM was installed successfully, joined to the domain, and the only Windows role enabled so far is File Server role. No additional applications have been installed.

The issue is that CPU usage remains unusually high compared to what I would expect from an idle file server. Norm interaction with the VM (RDP session, opening Server Manager, browsing folders, etc.) causes noticeable CPU spikes.

Below is my Environment
  • Proxmox VE: 9.x
  • Guest OS: Windows Server 2025
  • CPU: Intel Xeon Gold 6252
  • vCPU assigned: 48
  • Storage: HPE MSA 2050 SAS SAN via multipath
  • Storage type: LVM-thin
  • Disk controller: VirtIO SCSI
  • BIOS: OVMF (UEFI)
  • Machine type: Q35

Troubleshooting completed​

  • Installed latest VirtIO drivers.
  • Tested different VirtIO driver versions now using virtio-win 0.1.271 (as i know 0.1.285 has a issue with server 2025)
  • Tested different CPU types:
    • x86-64-v2-AES
    • x86-64-v3
    • x86-64-v4
  • Tested different disk cache options.
  • Enabled IO Thread.
  • Verified SAN performance and health.

The same underlying SAN is currently used by our VMware environment without any performance issues.
At this stage I am trying to understand if there are any recommended Proxmox settings specifically for Windows Server 2025 guests.

Are there any known issues or recommended settings around:
  • Windows Server 2025 and KVM/QEMU
  • VirtIO driver versions
  • CPU type selection
  • Hyper-V enlightenments
  • VBS / Core Isolation
  • QEMU Guest Agent
  • NUMA configuration
Any advice would be appreciated as we are evaluating Proxmox for our production environment.


Thanks.

Bash
 
hmmm there currently known issue explained in:
mariol

Post in thread 'Windows Server 2025 CPU suddenly at 100%'

Since several users have encountered this issue, these two workarounds have proven to be a current solutions:

The easiest option is to select a generic CPU for the VM, for example, x86-64-v2-AES [0].
The second option would be to create a custom CPU model based on the host and then remove the vmx-apicv-vid flag.
This should allow the VMs to work normally again. Feel free to test.


[0] https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_qemu_cpu_types
[1] https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_custom_cpu_types

Where its recommended to use CPU type other than host CPU. So you can roll out this and keep CPU type as x86-64-v2-AES.

Please share the vm config:
root@pve ~# qm config [VMID]


Also if you can share journalctl log from observed issue time from the pve host:
https://learning.lpi.org/en/learning-materials/102-500/108/108.2/108.2_02/
 
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In theory host with disabled nested virtualization shoukd get the best performance. x86-64-v2-AES is a quite old architecture and afik not even offically supported on recent windows systems. so I would definitively try out the others.
Following tool is quite handy to determine the maximum possible generic type: https://github.com/credativ/ProxCLMC
 
Are you actually using 48 vCPU? Or is that a typo? If you are using 48, that is probably the issue. I am not an expert, so I could be wrong, but overkilling the vCPU causes issues, from what I understand. In contrast, I have a 2025 file server with 4 vCPU that runs flawlessly and stays at 6-8%.
 
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No the host has 48 VCPU available I am only using 4 on the windows machine.

agent: 1
bios: ovmf
boot: order=scsi1;ide0;ide1
cores: 4
cpu: x86-64-v2-AES
efidisk0: DG_PH_A_OS1:vm-106-disk-0,efitype=4m,ms-cert=2023k,pre-enrolled-keys=1,size=4M
ide0: local:iso/virtio-win-0.1.271.iso,media=cdrom,size=709474K
ide1: local:iso/virtio-win-0.1.271.iso,media=cdrom,size=709474K
ide2: local:iso/virtio-win-0.1.271.iso,media=cdrom,size=709474K
machine: pc-q35-11.0+pve2
memory: 16096
meta: creation-qemu=11.0.0,ctime=1783614520
name: ICT-BUS-FS-01
net0: virtio=BC:24:11:37:39:10,bridge=vmbr1,firewall=1,tag=20
numa: 0
ostype: win11
scsi1: DG_PH_A_OS1:vm-106-disk-2,cache=writeback,iothread=1,size=200G
scsihw: virtio-scsi-single
smbios1: uuid=a6cfef4f-6b92-4ddd-b5ae-e73db1c6587d
sockets: 1
tpmstate0: DG_PH_A_OS1:vm-106-disk-3,size=4M,version=v2.0
vmgenid: aa29095f-22c3-4ba7-a096-e7f526aeb618
 
Can't comment, as I don't really use Windows Server 2025, but some general housekeeping:

mubbasher.hussain said:
boot: order=scsi1;ide0;ide1
Click to expand...
ide0 & ide1 are probably not bootable, so remove from boot options.

mubbasher.hussain said:
ide0: local:iso/virtio-win-0.1.271.iso,media=cdrom,size=709474K
ide1: local:iso/virtio-win-0.1.271.iso,media=cdrom,size=709474K
ide2: local:iso/virtio-win-0.1.271.iso,media=cdrom,size=709474K
Click to expand...
Remove 2 of those!

mubbasher.hussain said:
memory: 16096
Click to expand...
Thats 15.71875 GB. Use 16384 if you really want 16 GB.

mubbasher.hussain said:
cpu: x86-64-v2-AES
Click to expand...
Try host type - to at least check the issue.
 
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Is the high CPU usage showing up inside the Windows Guest (Task Manager), or on the Proxmox Host GUI?
If inside the Windows VM What specific process is consuming the CPU?

I would suggest to test to:
- Increase the assigned core to e.g 8 cores instead of 4 if possible.
- Test with CPU type host but important With hv_emcs enabled: Hardware -> Processors, Check Advanced options, scroll down and enable hv_emcs:
testing.png

My understand this what is reported on the fix for the aforementioned thread with issue with CPU type host:
https://lore.proxmox.com/all/20260714104201.71308-1-f.ebner@proxmox.com/T/#u
 
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mubbasher.hussain said:
No the host has 48 VCPU available I am only using 4 on the windows machine.

agent: 1
bios: ovmf
boot: order=scsi1;ide0;ide1
cores: 4
cpu: x86-64-v2-AES
efidisk0: DG_PH_A_OS1:vm-106-disk-0,efitype=4m,ms-cert=2023k,pre-enrolled-keys=1,size=4M
ide0: local:iso/virtio-win-0.1.271.iso,media=cdrom,size=709474K
ide1: local:iso/virtio-win-0.1.271.iso,media=cdrom,size=709474K
ide2: local:iso/virtio-win-0.1.271.iso,media=cdrom,size=709474K
machine: pc-q35-11.0+pve2
memory: 16096
meta: creation-qemu=11.0.0,ctime=1783614520
name: ICT-BUS-FS-01
net0: virtio=BC:24:11:37:39:10,bridge=vmbr1,firewall=1,tag=20
numa: 0
ostype: win11
scsi1: DG_PH_A_OS1:vm-106-disk-2,cache=writeback,iothread=1,size=200G
scsihw: virtio-scsi-single
smbios1: uuid=a6cfef4f-6b92-4ddd-b5ae-e73db1c6587d
sockets: 1
tpmstate0: DG_PH_A_OS1:vm-106-disk-3,size=4M,version=v2.0
vmgenid: aa29095f-22c3-4ba7-a096-e7f526aeb618
Click to expand...
That makes way more sense. As I said, I am new myself and have no idea why or how these are different, but your config is almost the same as mine, with a few exceptions. My efidisk is 512K instead of 4M, my machine is pc-q35-11.0 without the +pve2, and my scsihw is virtio-scsi-pci instead of virtio-scsi-single.
 
crawrj said:
That makes way more sense. As I said, I am new myself and have no idea why or how these are different, but your config is almost the same as mine, with a few exceptions. My efidisk is 512K instead of 4M, my machine is pc-q35-11.0 without the +pve2, and my scsihw is virtio-scsi-pci instead of virtio-scsi-single.
Click to expand...
Is yours working fine without any CPU spikes?
 
When idle, yes, I don't get any significant spikes. It hovers between 6 and 8%. When I do things on the server, it spikes for a moment and then calms down, but it never stays there for more than a few seconds. For example, when I launch the console, it spikes to 20% for a couple of seconds. Then, when I logged into the guest, it spiked to 100% for 2 seconds, then 50% for a few more seconds, then hovers around 15% from there. That's from a fresh login after a VM reboot. If I login after it has already been up and logged into it never spikes above 15%. I just did this so we have a live test.
 
Seeing the same behavior.
CPU usage is reported in Proxmox between 8% to 11% when idle.
taskmanager in the VM reports below 1%.

Server2025
Ryzen 4800h CPU

Happens both with default "x86-64-v2-AES" as well as "host" CPU type.
"hv_emcs" is not available on AMD.
 
whatthefiswrongwithcluste said:
Seeing the same behavior.
CPU usage is reported in Proxmox between 8% to 11% when idle.
taskmanager in the VM reports below 1%.

Server2025
Ryzen 4800h CPU

Happens both with default "x86-64-v2-AES" as well as "host" CPU type.
"hv_emcs" is not available on AMD.
Click to expand...
Do you also only observe this with Windows Server 2025 ?
Please share vm config:
qm config [VMID]


The known issue is mentioned for Intel:
https://pve.proxmox.com/wiki/Roadmap#9.2-known-issues
mariol

Post in thread 'Windows Server 2025 CPU suddenly at 100%'

Since several users have encountered this issue, these two workarounds have proven to be a current solutions:

The easiest option is to select a generic CPU for the VM, for example, x86-64-v2-AES [0].
The second option would be to create a custom CPU model based on the host and then remove the vmx-apicv-vid flag.
This should allow the VMs to work normally again. Feel free to test.


[0] https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_qemu_cpu_types
[1] https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_custom_cpu_types
 
longer said:
Do you also only observe this with Windows Server 2025 ?
Please share vm config:
qm config [VMID]


The known issue is mentioned for Intel:
https://pve.proxmox.com/wiki/Roadmap#9.2-known-issues
mariol

Post in thread 'Windows Server 2025 CPU suddenly at 100%'

Since several users have encountered this issue, these two workarounds have proven to be a current solutions:

The easiest option is to select a generic CPU for the VM, for example, x86-64-v2-AES [0].
The second option would be to create a custom CPU model based on the host and then remove the vmx-apicv-vid flag.
This should allow the VMs to work normally again. Feel free to test.


[0] https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_qemu_cpu_types
[1] https://pve.proxmox.com/pve-docs/pve-admin-guide.html#_custom_cpu_types
Click to expand...

Yes, this is specifically with Server 2025.
I have not encountered this with other operating systems.

My CPU usage is also not 100% but rather constantly around 8% to 11% despite the VM being idle.

qm config:
Code: 
agent: 1
bios: ovmf
boot: order=scsi0;ide2;ide0;net0
cores: 2
cpu: host
efidisk0: local-lvm:vm-100-disk-0,efitype=4m,ms-cert=2023k,pre-enrolled-keys=1,size=4M
ide0: TrueNAS:iso/virtio-win-0.1.285.iso,media=cdrom,size=771138K
ide2: TrueNAS:iso/en-us_windows_server_2025_updated_sep_2025_x64_dvd_6d1ad20d.iso,media=cdrom,size=7299130K
machine: pc-q35-11.0+pve2
memory: 2048
meta: creation-qemu=11.0.2,ctime=1785694788
name: dc01
net0: virtio=BC:24:11:65:A1:7C,bridge=vmbr0,firewall=1
numa: 0
ostype: win11
scsi0: local-lvm:vm-100-disk-1,discard=on,iothread=1,size=80G,ssd=1
scsihw: virtio-scsi-single
smbios1: uuid=2d71d82f-06a2-406f-af73-b342085bae47
sockets: 1
tablet: 0
tpmstate0: local-lvm:vm-100-disk-2,size=4M,version=v2.0
vmgenid: aaf78332-4d71-4be2-8f89-c7cdd6b55cce
 
whatthefiswrongwithcluste said:
Yes, this is specifically with Server 2025.
I have not encountered this with other operating systems.

My CPU usage is also not 100% but rather constantly around 8% to 11% despite the VM being idle.

qm config:
Code: 
agent: 1
bios: ovmf
boot: order=scsi0;ide2;ide0;net0
cores: 2
cpu: host
efidisk0: local-lvm:vm-100-disk-0,efitype=4m,ms-cert=2023k,pre-enrolled-keys=1,size=4M
ide0: TrueNAS:iso/virtio-win-0.1.285.iso,media=cdrom,size=771138K
ide2: TrueNAS:iso/en-us_windows_server_2025_updated_sep_2025_x64_dvd_6d1ad20d.iso,media=cdrom,size=7299130K
machine: pc-q35-11.0+pve2
memory: 2048
meta: creation-qemu=11.0.2,ctime=1785694788
name: dc01
net0: virtio=BC:24:11:65:A1:7C,bridge=vmbr0,firewall=1
numa: 0
ostype: win11
scsi0: local-lvm:vm-100-disk-1,discard=on,iothread=1,size=80G,ssd=1
scsihw: virtio-scsi-single
smbios1: uuid=2d71d82f-06a2-406f-af73-b342085bae47
sockets: 1
tablet: 0
tpmstate0: local-lvm:vm-100-disk-2,size=4M,version=v2.0
vmgenid: aaf78332-4d71-4be2-8f89-c7cdd6b55cce
Click to expand...

I think this is a different concern.
The CPU usage reported in Proxmox between 8% to 11% when idle while taskmanager in the VM reports below 1% might be expected in certain cases, I found related thread below [0]:
O

Thread 'High VM-EXIT and Host CPU usage on idle with Windows Server 2025'

Good evening, I have been pouring over an interesting problem and it seems like I need some deep insight. Windows Server 2025 Core has much more than expected overhead.

I am testing Windows Server 2025 Core (and Desktop) on Proxmox 8.3, with all paravirtualized features enabled and the agent installed.

On the VM all virt-io drivers have been installed and VBS has been explicitly disabled. The OS is newly installed, is completely idle, with no software installed, and running idle for quite some time, so no background or startup tasks.

I have tested various combinations of cpu flags...

For this current thread, the issue is (if I understand correctly), unusually high CPU usage inside the VM. Take a look at the mentioned thread [0] (or perhaps grab a coffee, as it is a long one).

You might also consider using virtio-win version 0.1.271, as the currently used 0.1.285 is known to have issues (I assume unrelated to increase CPU), check [1] for details.

[0] https://forum.proxmox.com/threads/h...sage-on-idle-with-windows-server-2025.163564/
[1] https://pve.proxmox.com/wiki/Window..._issues_with_IO-heavy_Windows_Server_2025_VMs
 
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